The present invention relates to methods for start up of pots or cells used in the electrolytic deposition production of metals. More specifically, the present invention relates to methods for protecting electrodes, from thermal shock and degradation by products of combustion during pre-heating of molten salt electrolysis cells. The present invention further relates primarily to protecting cathodes and other carbon cell components during pre-heating.
Aluminum is produced conventionally by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures between about 900xc2x0 C. and 1000xc2x0 C.; the process is known as the Hall-Heroult process. A Hall-Heroult reduction cell typically comprises a steel shell having an insulating lining of refractory material, which in turn has a lining of carbon that contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate that forms the cell bottom floor. The carbon lining and cathode substrate have a useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminum as well as intercalation of sodium, which causes swelling and deformation of the cathode carbon blocks and ramming mix. In addition, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides. Anodes are at least partially submerged in the bath.
In operation, the conventional cell contains an electrolytic, molten cryolite-based bath in which alumina is dissolved. A molten aluminum pool acts as the cathode. A crust of frozen electrolyte and alumina forms on top of the bath and around the anode blocks. As electric current passes through the bath between the anode and cathode surfaces, alumina is reduced to aluminum, which is deposited in the pad of molten metal.
Electrolytic reduction cells must be heated from room temperature to approximately the desired operating temperature before the production of metal can be initiated. Heating should be done gradually and evenly to avoid thermal shock, which can in turn cause breakage or spalling of the anodes, sidewalls and cathode blocks. The heating operation minimizes thermal shock to the lining and the electrodes upon introduction of the molten electrolyte to the cell.
Preheating of cells is typically performed by either a gas preheat or by resistor block. The gas pre-heating step results in the generation of products of combustion (POC), such as at least one of CO, CO2, and H2O, which can be deleterious to the cathodes and anodes. CO can reduce the oxides in the anodes, eventually leading to corrosion. CO2 and H2O can also oxidize the carbon in the cathode block, producing CO+H2. CO2 and H2O can oxidize metallic constituents of the anode, again leading to corrosion. It is therefore desirable to protect the cathodes and anodes from all constituents present in POC.
Aluminum electrolysis cells have historically employed carbon anodes on a commercial scale. The energy and cost efficiency of aluminum smelting can be significantly reduced with the use of inert, non-consumable, and dimensionally stable anodes. Use of inert anodes rather than traditional carbon anodes allows a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also realized because inert anodes produce essentially no CO2 or CF4 emissions. Some examples of inert anode compositions are provided in U.S. Pat. Nos. 4,374,050; 4,374,761; 4,399,088; 4,455,211; 4,582,585; 4,584,172; 4,620,905; 5,279,715; 5,794,112; 5,865,980; and 6,126,799 assigned to Alcoa Inc. Inert anodes can undergo thermal shock if heated or cooled too quickly, and can also undergo degradation from exposure to POC.
It is well known to apply coatings to anodes to protect them during electrolysis; to provide a wetted bottom cathode surface or to repair or renew anode surfaces, as taught by U.S. Pat. Nos. 5,069,771 and 5,340,448. Cathodes have heretofore been protected according to the teachings of U.S. Pat. No. 5,492,604, using an aluminum paint containing at least aluminum powder and an organic binder, and a refractory metal source preferably selected from the elements titanium, zirconium, hafnium and their oxides to provide a metal-wettable coating. Aluminum foil appears to have been used as exterior cathode protection during heat-up. Also, as taught in U.S. Pat. No. 5,534,130, carbonaceous electrolytic cell components were treated with a solution of one or more phosphates of aluminum and one or more colloidal carriers. However, what is needed is a coating useful for pre-heat start-up of a new pot without damaging new carbon cathode cell bottoms or new cermet anodes already installed in the pot shell. What is needed is a new way to maintain the condition and surface of the original cathode and anode material so they do not need to be renewed or repaired.
In addition to attack of electrodes during pot startup, difficulties in operation can arise from the accumulation of undissolved alumina sludge on the surface of the carbon cathode material lying beneath the aluminum pool; this sludge can form insulating regions on the cell bottom. Penetration of cryolite, sodium and aluminum through the carbon body can result in deformation of the cathode carbon blocks and can also cause displacement of the blocks; this can give rise to other problems that negatively impact output of the cell.
A major drawback of carbon as a cathode material is that it is not wetted by aluminum. This necessitates maintaining a deep pool of aluminum (at least 100-250 mm thick) in order to ensure a certain protection of the carbon blocks and an effective contact over the cathode surface. Electromagnetic forces create waves in the molten aluminum and, to avoid short-circuiting with the anode, the anode-to-cathode distance ACD must be kept at a safe minimum value, usually 40 to 60 mm. For conventional cells, there is a minimum ACD below which the current efficiency drops drastically, due to short-circuiting between the aluminum pool and the anode.
The present invention is directed to methods for protecting electrodes during start up of metal producing electrolytic cells, or xe2x80x9cpotsxe2x80x9d as they are referred to in the art, which utilizes a gas pre-heat. The entire pot and all its components should be carefully heated to temperature, and the gas atmosphere around the anodes and cathodes should be carefully controlled. More specifically, the gas atmosphere during heat-up should be maintained in a slightly oxidizing state, with enough oxidation to minimize carbon deposit or soot, but with not so much oxidation that exposed carbon will be attacked. Inert anodes are protected both from thermal shock and from products of combustion (POC) according to the present methods.
The present invention is directed to the application of a protective coating to the carbon cathode substrate forming the cell bottom floor, to protect it during gaseous pre-heating conditions. The coating is comprised of two or more layers including at least one layer selected from the group consisting of refractory material, metal, metal alloy and an outer carbon layer. Application of the coating minimizes early failure of the cathode and also, indirectly the inert anode, due to the effects of POC gases. A protective coating can optionally be applied to other carbon components prior to the pre-heating step; this serves to minimize reaction between the gas used in the preheat step and the carbon surfaces, thereby allowing for better control of the fuel to air ratio and better temperature control during heat-up. Better control of the fuel to air ratio permits better control of the oxygen partial pressure in the POC gas. Reduction or oxidation of the anode surface can be eliminated or controlled by careful control of the fuel to air ratio.